Quantification and comparison of teleost scleral cartilage development and growth.

The ocular skeleton is composed of the scleral cartilage and the scleral ossicles. Teleost scleral cartilage is composed of a single layer of chondrocytes embedded in the sclera of the eye. The teleost scleral cartilage ring can vary in depth across teleost families and species, from a narrow ring a few cells wide to a deeper ring that resembles a cup and surrounds the entire sclera. However, very little research has been conducted on the development and morphology of teleost scleral cartilage. Thus, this study aims to characterize the development of the scleral cartilage in the zebrafish and Mexican tetra, with respect to the timing of emergence, depth throughout development, and positioning within the eye. We hypothesized that the scleral cartilage would first emerge in the scleral tissue closely abutting the ora serrata, and that growth would proceed in an anterior-to-posterior direction, resulting in differences in scleral cartilage depth between different fish species. We found that the scleral cartilage ring does not develop uniformly along its circumference, and that its relationship to the ora serrata varies between the rostral and caudal regions. Furthermore, distinct differences in the growth trajectory of the scleral cartilage indicate that the deep scleral cartilage of the Pachón cavefish is the result of both decreased eye size and prolonged cartilage growth. A significant difference in the size of the scleral chondrocytes was also noted. Overall, this study provides the first characterization of early scleral cartilage development in teleost fish and indicates that some aspects of scleral cartilage development and morphology are highly conserved while others are not.